Centrifugal compressors are often used in gas turbomachinery to compress and direct a pressurized air or gas flow to the gasifier section in a gas turbine engine. In the gasifier section a combustion process dramatically heats the gas flow which is then exhausted across one or more turbine stages to create rotational mechanical power and/or thrust through exhaust of the gas flow. Great care must be taken in the design and configuration of such centrifugal compressors to provide sufficient operation at the desired power speed while avoiding surge or stall of the compressor. Characteristically, the surge margin for compressors is an important criteria in their design and operation.
Many applications of modern gas turbomachinery such as gas turbine engines may optimally require operation of the compressor at two different design points, one point being the normal full power setting for the engine and a second, part speed design point, for lower power or cruise operations. The purpose of operating at two different design points is one of efficiency and minimization of the specific fuel consumption of the engine when considering its entire design operational envelope.
Difficulties are recognized in providing a compressor with such dual design point operation inasmuch as the compressor obviously must be designed to produce adequate flow at the required high speed or full power condition. When such compressor is then operated at reduced speed and power conditions the impeller blade leading edge will be operating at high incidence angles relative to the air intake. This tends to create considerable pressure losses at the part-power operation due to these high incidence angles, and also greatly reduces the surge margin of the centrifugal compressor when operating at this part-power design point.
Various prior configurations are known which attempt to take advantage of the known "pressure reversal" which occurs at a downstream point on the compressor impeller when operating at either the part speed or full speed design points. Examples include U.S. Pat. Nos. 4,248,566 and 2,405,282.